Two-terminal PTC (Positive Temperature Coefficient of resistance) and NTC (Negative Temperature Coefficient of resistance) variable resistance devices have been known and used since the 1970's.
Fuse and circuit breaker replacement PTC devices have been used in batteries, motor protectors, and telecommunications equipment for many years. The advantage of this technology over both fuses and circuit breakers is that such a device is self-resetting when the cause of the voltage or current overload is removed and the device cools down. For example, a child physically holds an electric window down in a car, while simultaneously pushing the “close” button for the window. Objectionably high current is produced in the motor circuit, which could burn out the motor. This increased current trips a PTC device which has been inserted in the electrical circuit that controls the window, limiting the current, and preventing burnout of the motor. The child releases the “close” button and stops physically restraining the window, the PTC device cools down, and normal operation of the electrical window is then restored. This technology can be used to prevent overheating of lithium ion batteries and destructive currents in computer circuits, and, as mentioned above, can protect motors that control electrical windows, electrical seats, electrical windshield wipers, and other devices in cars.